1. Field of the Invention
The present invention relates to an image sensing apparatus using an image sensor and to a control method therefor, and in particular, to an image sensing apparatus which has a focus detection function based on sensed images and a live-view function, and to a control method therefor.
2. Description of the Related Art
As an auto-focusing (AF) method to be used for image sensing apparatuses such as digital cameras and digital video cameras, methods based on contrast of a sensed image, such as a so-called hill-climbing search, are well known. Hereinafter, auto-focusing with the use of sensed images is referred to as image-sensing plane AF.
On the other hand, the functions unique to image sensing apparatuses that use image sensors include functions referred to as an electronic viewfinder or live view, in which the display device is allowed to function as a viewfinder by shooting a large number of images, for example, 30 frames or 60 frames of images per second with the use of an image sensor, and consecutively displaying the sensed images on the display device.
In the case of achieving live view, the number of images read from an image sensor per predetermined time period (shots per predetermined time period), that is, the frame rate, constrains the upper limit of the charge accumulation time at the image sensor, that is, the minimum speed of an electronic shutter.
Therefore, the brightness control for images to be used for live view (referred to as live-view images) is exercised by adjusting not only the charge accumulation time but also the aperture and the gain. As described above, the frame rate for live-view images is typically 30 frames/second, or double that at 60 frames/second. Further, as a method for brightness (exposure) control for live-view images, there is a typical method in which the charge accumulation time and gain are changed with the maximum aperture fixed. In this specification, such a method is referred to as normal control.
In a case in which image sensing is carried out with the use of an image sensing apparatus using an image sensor without synchronism in accumulation, such as a CMOS image sensor, under a light source with its brightness varying cyclically (flicking light source), typified by a fluorescent lamp, the sensed images are known to be affected by changes in brightness of the light source.
Therefore, when live view is carried out under a flicking light source with an image sensing apparatus using a CMOS image sensor, horizontal-striped line flickers will occur in live-view images as shown in FIG. 5, which is undesirable. The number of line flickers is unambiguously determined by the reading cycle (the interval in a vertical direction) for and the number of read lines for the CMOS image sensor.
Thus, during execution of live view, it is necessary to check if flickers occur in live-view images and remove or reduce the flickers, if any.
For example, in a case in which flickers are attributed to a fluorescent lamp, the flickers have a frequency corresponding to the frequency of the commercial power supply. Further, for example, in a case in which the frequency of the commercial power supply is 50 Hz, as for flickers attributed to the fluorescent lamp, line flickers are known to be able to be removed from live-view images by setting the charge accumulation time to 10 ms, 20 ms, 30 ms, or the like (Japanese Patent Laid-Open No. S63-308484).
More specifically, in a case in which it is necessary to remove flickers, brightness control for live-view images is exercised by discretely setting the accumulation time and adjusting the aperture and the gain, unlike normal control. In this specification, such a method for brightness control is referred to as flicker-reduction control.
However, when image-sensing plane AF is carried out with the use of live-view images, in a case in which flicker-reduction control is exercised by controlling the accumulation time, it is necessary to control the aperture during the live view with the accumulation time. Furthermore, the degree of freedom of the accumulation time itself is constrained. Then, with the aperture controlled, the depth of field will vary, or the degree of freedom for the control of the aperture will be constrained.